Prepare theme for child theme + enqueue admin stylesheet + change hook for analytics

It would be nice for users to be able to use this wonderful framework as parent theme (that's why I added a lot of `if(!function_exists())`). Doing this allows to easily override parent functions in a child theme.

The Google Analytics script is asynchronous so you can put it in head section (better tracking)
Merge pull request #11 from jmlapam/patch-5

Prepare theme for child theme + enqueue admin stylesheet + change hook f...